Propylene impact copolymers (ICP's) are commonly used in a variety of applications where strength and impact resistance are desired such as molded and extruded automobile parts, household appliances, luggage and furniture.
Propylene homopolymers or propylene based random copolymers having high crystallinity are often unsuitable for such applications by themselves because they are too brittle and have low impact resistance, whereas propylene impact copolymers are specifically engineered for applications such as these.
Propylene impact copolymers are typically an intimate mixture of a continuous phase of crystalline propylene homopolymer or random copolymer and dispersed rubbery phase of ethylene-propylene copolymer. In general the continuous phase is known to provide properties such as stiffness and the dispersed phase provides impact resistance properties.
In general, it has been observed that the properties of stiffness, and impact resistance trend in opposite directions such that as stiffness increases, impact resistance decreases and vice versa. It would be desirable to develop compositions which exhibit improved stiffness without degrading the impact resistance performance.
The present invention relates to such a composition. Specifically, one aspect of the present invention is a polypropylene impact copolymer comprising: a matrix and a dispersed phase. The matrix comprises a polypropylene homopolymer or a propylene/alpha-olefin random copolymer which comprises more than 50 wt. % of units derived from propylene monomer. The matrix should have a relatively high crystallinity, preferably 50% or greater. The polypropylene homopolymer or a propylene/alpha-olefin random copolymer preferably has a MWD between 4 and 8, such as typically obtained using Ziegler-Natta catalysts. The dispersed phase in the impact copolymer comprises an ethylene-propylene copolymer which comprises from 45 to 70 wt. % of units derived from an ethylene monomer. Preferably the dispersed phase comprises from 20 to 50 percent by weight of the polypropylene impact copolymer